The present invention relates to compound semiconductor devices such as a HEMT (high electron mobility transistor).
A HEMT structure is now drawing much attention, which is a field-effect transistor utilizing a two-dimensional electron gas accumulated in a heterojunction interface. The HEMT structure is illustrated in FIG. 6. An undoped semiconductor layer 2 is formed on a substrate 1, and a doped semiconductor layer 3 having an electron affinity smaller than the layer 2 is formed on the layer 2. Agate electrode 4 is formed on the doped semiconductor layer 3. A source electrode 6 and a drain electrode 7 are formed on cap layers 5 that are formed on the doped semiconductor layer on both sides of the gate electrode 4.
In this HEMT structure, all the donor impurities added to the smaller electron affinity doped semiconductor layer 3 are ionized, and electrons generated by this ionization are accumulated in the heterojunction interface with the larger electron affinity undoped semiconductor layer 2 to form a two-dimensional electron gas 8. The distribution of the electron gas 8 can be controlled by a voltage applied to the gate electrode 4, to enable control of a current flowing between the source and drain.
FIGS. 7(a) and 7(b) are energy band diagrams showing electron distributions D when no voltage is applied to the gate electrode 4 (FIG. 7(a)), i.e., the device is not in use, and when a certain negative voltage is applied (FIG. 7(b)). As shown in these figures, the distribution D of the two-dimensional electron gas 8 assumes a mountain-like shape having some spread which is larger when the negative voltage is applied to the gate electrode 4. The degree of expansion depends on the magnitude of the negative voltage.
In other words, the center of gravity P of the electron distribution D goes farther away from the heterojunction interface as the negative voltage increases. Particularly in low-noise HEMTs in which a negative voltage is applied to the gate electrode to reduce the current, this phenomenon will deteriorate the electron controllability and make it difficult to realize satisfactory low-noise characteristics.